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pg_freespacemap.c | ||
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README.pg_freespacemap |
Pg_freespacemap - Real time queries on the free space map (FSM). --------------- This module consists of a C function 'pg_freespacemap()' that returns a set of records, and a view 'pg_freespacemap' to wrapper the function. The module provides the ability to examine the contents of the free space map, without having to restart or rebuild the server with additional debugging code. By default public access is REVOKED from both of these, just in case there are security issues lurking. Installation ------------ Build and install the main Postgresql source, then this contrib module: $ cd contrib/pg_freespacemap $ gmake $ gmake install To register the functions: $ psql -d <database> -f pg_freespacemap.sql Notes ----- The definition of the columns exposed in the view is: Column | references | Description ----------------+----------------------+------------------------------------ blockid | | Id, 1.. max_fsm_pages relfilenode | pg_class.relfilenode | Refilenode of the relation. reltablespace | pg_tablespace.oid | Tablespace oid of the relation. reldatabase | pg_database.oid | Database for the relation. relblocknumber | | Offset of the page in the relation. blockfreebytes | | Free bytes in the block/page. There is one row for each page in the free space map. Because the map is shared by all the databases, there are pages from relations not belonging to the current database. When the pg_freespacemap view is accessed, internal free space map locks are taken, and a copy of the map data is made for the view to display. This ensures that the view produces a consistent set of results, while not blocking normal activity longer than necessary. Nonetheless there could be some impact on database performance if this view is read often. Sample output ------------- regression=# \d pg_freespacemap View "public.pg_freespacemap" Column | Type | Modifiers ---------------+---------+----------- blockid | integer | relfilenode | oid | reltablespace | oid | reldatabase | oid | relblocknumber | bigint | blockfreebytes | integer | View definition: SELECT p.blockid, p.relfilenode, p.reltablespace, p.reldatabase, p.relblocknumber, p.blockfreebytes FROM pg_freespacemap() p(blockid integer, relfilenode oid, reltablespace oid, reldatabase oid, relblocknumber bigint, blockfreebytes integer); regression=# SELECT c.relname, m.relblocknumber, m.blockfreebytes FROM pg_freespacemap m INNER JOIN pg_class c ON c.relfilenode = m.relfilenode LIMIT 10; relname | relblocknumber | blockfreebytes ------------------------+----------------+---------------- sql_features | 5 | 2696 sql_implementation_info | 0 | 7104 sql_languages | 0 | 8016 sql_packages | 0 | 7376 sql_sizing | 0 | 6032 pg_authid | 0 | 7424 pg_toast_2618 | 13 | 4588 pg_toast_2618 | 12 | 1680 pg_toast_2618 | 10 | 1436 pg_toast_2618 | 7 | 1136 (10 rows) regression=# Author ------ * Mark Kirkwood <markir@paradise.net.nz>